The present invention generally relates to a diode unit, and more particularly, to a laser diode for use in an electrical or electronic equipment.
Conventionally, in the field of laser diodes, there have been employed a so-called can-seal type and a unit type. As shown in a cross sectional view of FIG. 4, one example of such known unit type laser diodes generally includes a substrate 42, and a laser diode chip 49 die-bonded on said substrate 42 through a sub-mount 43, with a photo-diode being incorporated in the sub-mount 43 as a monitor element 47.
The monitor element 47 referred to above is arranged to receive laser light emitted from a rear cleavage face 49b of the laser diode chip 49. Based on the light receiving current (i.e. monitor current) of the monitor element 47, driving current for the laser diode chip 49 is controlled by an APC (automatic phase control) circuit so as to allow laser light output of the laser diode chip 49 to reach a predetermined value.
However, when the laser diode is made into an open type to be used in a state where it is exposed to atmosphere, there may arise such a case where an amount of light incident upon the monitor element 47 is reduced by dew formation or adhesion of dust, etc. on the surface of the monitor element, thus, making it impossible to fully detect the monitor current. In the case as referred to above, there is a possibility that the driving current continues to be increased by the APC circuit, and undesirably breaks down the laser diode chip 49 finally.
Meanwhile, in the unit type laser diode, there is such a fundamental problem that the incident angle of the laser light upon the monitor element 47 is originally small, and a sufficient amount of laser light cannot be directed into the monitor element 47.
Therefore, a solid state waveguide 50 made of a light transmitting (or semi-light transmitting) resin is provided to connect a space between a rear cleavage face 49b of the laser diode chip 49 and the surface of the monitor element 47. Within the above solid state waveguide 50, laser light is subjected to total reflection by a difference in the refractive indices at a boundary face between the resin and air so as to be incident upon the monitor element 47.
On the sub-mount 43, an aluminum wiring pads 45 conducted to the laser diode chip 49 and the monitor element 47 is formed so as to be bonded to leads (not shown) on a flexible circuit 52 by a wire W.
In the conventional laser diode as described above, the shape of the solid state waveguide 50 has influence upon the light amount to be received by the monitor element. Since the solid state waveguide 50 is formed by applying resin in a liquid state into the space referred to above for subsequent hardening, the shape thereof differs one by one, and there may be a case where an angle of incidence of the laser light with respect to the boundary face (surface of the solid state waveguide means) between the resin and air becomes large, and thus, part of the laser light is transmitted outside the solid state waveguide 50. In such a case, there is a problem that the amount of light incident upon the monitor element 47 is reduced, and a sufficiently large monitor current cannot be obtained.
Another disadvantage of the conventional laser diode is such that, when the laser diode is formed into an open construction without covering, the surface of the solid state waveguide 50 is exposed to the atmosphere for a long period, and the state of its surface i.e. of its boundary face is altered, so as to vary light amount incident upon the monitor element 47 with consequent variation in the monitor current. In some cases, such a problem may also result from dew formation, adhesion of dust, etc., on the surface of the solid state waveguide 50.